In Kafka's story "A Report to an Academy", an ape called Red Peter delivers a lecture to a learned society in which he gives an account of the life he led before he acquired human ways. Captured on the Gold Coast (now Ghana), Red Peter was transported in a cage to Hamburg. In that city, he reports, he faced two alternatives - the zoological gardens or the variety stage. Life in the zoological gardens meant only another cage, so he chose the stage. It was not easy to get into the variety hall, but once there Red Peter was an enormous success. Soon he learnt to talk like a human, and it was not long before he achieved what he termed "the cultural level of an average European". His stage performances enabled him to enjoy a distinctly human way of life. As he described it in his report to the academy: "When I come home late at night from banquets, from scientific receptions, from social gatherings, there sits waiting for me a half-trained little chimpanzee, and I take comfort from her as apes do."

Kafka's story is cited in J M Coetzee's The Lives of Animals, a profound fictional meditation on the contradictions that beset our attitudes to other animal species. The story of Red Peter is a fantastical version of the fate that befell many apes, and - as one of Coetzee's characters notes - there were real-life prototypes of Red Peter. In 1912, the Prussian Academy established a research centre on the island of Tenerife to study the mental powers of apes; and in 1917, the director of the centre, Wolfgang Kohler, published some of the results of this in his celebrated study The Mentality of Apes. Like Red Peter, Kohler's apes underwent a period of training designed to induce them to adopt human ways. Among the pedagogic methods used was slow starvation, with the apes being repeatedly shown and denied food until they developed something resembling human faculties.

It is not clear how researchers today would assess the results of this experiment, but Kohler - one of the founders of cognitive psychology - seems to have seen it as a success, noting with satisfaction how the captive chimpanzees ran in a circle round their compound, some draped in old strips of cloth and others carrying pieces of rubbish, "for all the world like a military band".

Kohler's experiments were cruel and demeaning to the animals on which they were inflicted, but they are chiefly notable for the deep confusion they exhibit in his - and our - view of our closest evolutionary kin. We have come to view apes as proto-humans, yet we subject them to treatment we would not dream of inflicting on members of our own species. If apes were not similar to us in important respects, many of the experiments to which they are subjected would be impossible or pointless. Few now deny that apes share much of our intellectual and emotional inheritance. They have many of our own capacities and vulnerabilities: they can think and plan, and they feel fear and love. Without these similarities, Kohler's experiments would have been unfeasible and could not have yielded the knowledge he was seeking. Yet these very similarities undercut the ethical basis of such experimentation.

We do not put humans into captivity and starve them in order to test their intellectual abilities because we know that such treatment would cause severe suffering. How can we justify such experimentation on apes, knowing that it can work only to the extent that their capacities - including the capacity to suffer - are much like our own? Can there be any compelling ethical defence of using creatures so like ourselves in ways that we would find unbearable? Or is the answer that the animals used in such experiments are simply unfortunate - that we have them in our power and their suffering is a regrettable but unavoidable result of our using them for our benefit?

The last of these options appears to have been taken by a recent spokesman for Cambridge University. Responding to protests against plans to establish a primate research centre there, he observed that it is an unfortunate fact that only primates have brains like our own. The implication is that it is precisely because apes have many of the capacities of humans that they are used for experimentation. It is true that experimenting on primates is a productive research technique; but if their similarities with us justify using apes in this way, it would surely be even more effective to use humans. The argument for experimenting on primates leads inexorably to the conclusion that it is permissible - in fact, preferable - to experiment on humans.

Quite rightly, the idea that humans should be used in painful or dangerous medical experiments evokes intense moral horror; but this has not always been so. Powerless and marginal people - in prisons and mental hospitals, for example - have in the past often been used as guinea-pigs, and it is all too easy to imagine the forcible use of humans in scientific research practised on a far wider scale. The Nazis saw nothing wrong in subjecting members of what they considered to be inferior populations to the most horrible experiments; and there can be little doubt that had the outcome of the Second World War been different, the use of humans for scientific research would have been institutionalised across Europe. No doubt it would have been condemned by a dedicated few, but the historical experience of occupied Europe suggests that the majority of the population would have accepted the practice.

It will be objected that there is a vital difference between using animals for scientific research and using humans: humans have the capacity for consent, whereas animals do not. It is true that adult humans can express their wishes to other humans in ways that even our closest animal kin cannot; but consent is not the heart of the matter. Even if they agreed, it would be morally intolerable to use prison inmates in dangerous medical experiments. No form of consent they might give could make the injury done to them less real; it would only reflect their powerlessness. Similarly, it is not the inability of human infants to give their consent that justifies an absolute ban on experimenting on them. It is the terrible damage we would inflict on them merely to produce benefits for ourselves.

The same is true of experiments on animals. It is not the capacity for consent that is most relevant, but the capacity for suffering. I am no Utilitarian, but Jeremy Bentham hit the spot when he wrote of animals that the crucial question is not "Can they speak?". Rather, it is "Can they suffer?".

At this point, those who support animal experimentation have a habit of wheeling out some extremely familiar arguments. Animals lack the capacity for personal autonomy, they tell us, and so cannot recognise duties to others. For the same reason, they cannot have rights. Humans have the power of choice, and this entitles them to a moral status denied to other animal species.

We hear this tired refrain whenever the subject of animals is discussed, but it is significant not so much for any intellectual content it may have, but for what it shows about the lingering influence of religious belief. If you are a Christian, it makes perfect sense to think of humans as standing in a different category from other animals. Humans have free will and an immortal soul, and these attributes confer an incomparable importance on human life. No doubt we should refrain from cruelty to other creatures, but they have no claim to value in their own right; they are instruments for achieving human ends. Humans have dominion over animals because humans alone are made in the image of God.

Secular thinkers find it extremely difficult to come up with reasons for thinking that the human species has some kind of unique standing in the world. Darwin showed that we share a common lineage with other animals, and subsequent genetic research has shown the closeness of these evolutionary links. Insofar as humans do have morally relevant attributes that other animals lack, it is right to treat them differently. But within a purely secular perspective there can be no good reason for thinking the human species is supremely valuable.

In the context of their beliefs about animals, as in many other areas, secular humanists parrot a Christian hymn of human uniqueness. They prattle on about the supreme value of human personality as if it were a self-evident truth. Yet it is not accepted in most of the world's religions, and is strikingly absent in some - such as Buddhism - that have never thought of other species as mere instruments of human purposes. Secular humanists are adopting the anthropocentric viewpoint of Christianity - while abandoning the theistic belief-system from which it sprang, and without which it is meaningless.

Once Christianity and humanism have been set aside, it becomes clear that the chief difference between humans and other animals is simply that humans have acquired enormous power. In evolutionary terms, the human species has been an astonishing success. In the space of a few thousand years, it has achieved a seeming mastery over its environment, which is reflected in a vast increase in human population. At the same time, humans have had a huge - and almost entirely harmful - impact on other animal species. The mass extinction of wildlife we are seeing throughout the world comes from the destruction of habitat - itself largely a result of rising human numbers. The damage done to the welfare of other animal species by human expansion is on an incomparably larger scale than anything that is done in scientific laboratories. This does not mean vivisection is unimportant - after all, no one thinks that since millions of people are slaughtered in wars it does not matter if some die as a result of murder - but it does mean that anyone who focuses on animal experimentation is missing the big picture.

The chief threat to animal welfare today comes from the unchecked expansion of homo rapiens. Wherever humans have entered a new environment the result has been a wave of extinctions. This is what happened when Polynesian settlers arrived in New Zealand a few hundred years ago, and it was the arrival of humans in North America around 12,000 years ago that accounts for the disappearance of about 70 per cent of its large mammals. Though some hunter-gatherer cultures may have reached a precarious balance with the natural world and a number of Buddhist peoples have displayed remarkable self-restraint, the history of human relations with other species is a record of almost unbroken rapacity. Wrecking the environment seems to be in the nature of the beast.

This may seem a despairing conclusion, but for anyone whose horizons are not confined to the human world, there are grounds for hope. While humans have enormous power over the environment, their capacity to control it is strictly limited. The present level of human population depends on maintaining high levels of industrial production, but global warming will prevent worldwide industrialisation on anything resembling the dominant western model. There is no way that eight billion people can have the lifestyle that has come into being in a few countries over the past century or so.

Most green thinkers believe that this transition can come about as a result of political action. By far the most effective way of limiting human numbers is giving women control of their own fertility. Making contraception and abortion universally available enhances human well-being and at the same time reduces the pressures that are destroying animal habitat. Population control should be central in any programme of transition to a more stable world and, in fact, many developing countries have population policies. However, the subject is surrounded with an aura of political incorrectness. As a result, it has become fashionable to talk as if a sustainable way of life can be achieved simply by shifting to a different economic system. In reality, finite resources impose insuperable limits on the growth of human numbers and the Earth's carrying capacity has probably already been breached.

One way or another, human expansion will be curbed; and a plausible scenario is that this will occur as a by-product of war. Globalisation supports the present high levels of human population, but its logic is to intensify the struggle for scarce natural resources. Resource wars, such as the two Gulf wars, look set to dominate the coming century. Such conflicts would be damaging to animals as well as humans, but because of their disruptive effect on the global supply chain, their impact on humans is likely to be much more severe. The end result could be a less crowded world in which other species have room to breathe. Homo rapiens is a ferociously destructive creature, but its capacity for self-destruction is even greater. The human behaviour that Wolfgang Kohler was so pleased to observe being parodied by his captive apes may yet prove to be the ultimate guarantee of animal liberation.